Sources for the various image files:

[biohazard.png]
URL: http://en.wikipedia.org/wiki/File:Biohazard.svg
License: public domain

[exit.png]
URL: http://commons.wikimedia.org/wiki/File:PublicInformationSymbol_EmergencyExit.svg
License: "The symbols presented in these guidelines can be freely used by
  anyone and can be accessed at the Web site (http//www.ecomo.or.jp) of the
  Eco-Mo Foundation. However, registering these symbols as your trademark or
  design can be a copyright infringement."

All other images are original and licensed under the same terms as the rest of
the game.